1. Field of the Invention
The present invention relates to various kinds of transistors such as a Thin Film Transistor (TFT) in a matrix type display apparatus such as a liquid crystal display. More particularly, the invention relates to a method of manufacturing a polycrystalline silicon layer which for use as the active layer of a transistor.
2. Description of the Related Art
There is increasing demand for display devices to display images with high resolution and high quality. To fulfill this requirement, Active Matrix Liquid Crystal Displays (AMLCD) using a thin film transistor as switching element for driving the crystal liquid are commonly used in liquid crystal displays.
Such AMLCDs with a TFT, commonly comprise an amorphous silicon TFT using an amorphous silicon region and a polycrystalline silicon TFT using a polycrystalline silicon film as an active layer of a thin film transistor as a channel region.
Among TFTs, the amorphous silicon TFT can be easily formed on a glass substrate at a low cost with a lower melting point because it may be formed at a lower temperature (e.g. 300xc2x0 C.). Additionally, the amorphous silicon TFT is advantageous to increasing the size of a display panel because it is easy to form a uniform amorphous silicon film over a large area. Therefore, the amorphous silicon TFTs are currently used for large LCDs.
However, because the mobility degree in the polycrystalline silicon film is higher than that in the amorphous silicon film, an xe2x80x9conxe2x80x9d current flows more in the polycrystalline silicon TFT and a sheet resistance (xe2x80x9conxe2x80x9d resistance) of the polycrystalline silicon TFT is lower. These characteristics allow the polycrystalline silicon TFT to show improved response characteristics and a better ability to drive a display. Accordingly, it is now understood that the polycrystalline silicon TFT is useful as switching elements in a high resolution, quality LCD. In addition, it is pointed out that the polycrystalline silicon TFT is useful as switching element to drive a liquid crystal for a larger LCD because a selection period (duty ratio) becomes shorter as the display becomes larger. Furthermore, since the polycrystalline silicon TFT uses the polycrystalline silicon film as an active layer, it can be used not only as a driving element for liquid crystals in a pixel portion, but also as a switching element constituting a logic circuit for a driver circuit. Furthermore, it is possible to form the driver elements for liquid crystals and the elements for a logic circuit on a same substrate in a same process. Accordingly, the polycrystalline silicon TFT is currently used for many small or middle sized LCDs which are required to have high resolution and high quality and to be small-sized, as a so-called a driver containing LCD in which a pixel portion and a driver portion are formed on a same substrate.
As mentioned above, a polycrystalline silicon TFT remains advantageous to use in larger displays because such a TFT enables a high resolution and quality LCD to be formed with space around the edges of the panel where the driver could contain. Such a panel would be lightweight.
To achieve these goals, it is necessary to form a polycrystalline silicon TFT on a cheap glass substrate having a low melting point (about 600xc2x0 C.) with a high yield rate comparable to that of an amorphous silicon TFT. Currently, however, it is difficult to form a polycrystalline silicon film having grains with an appropriate size at a temperature below a melting point of the glass substrate (about 600xc2x0 C.). Therefore, it is suggested that an amorphous silicon film first be formed on a substrate and then a polycrystalline silicon film be formed by polycrystallizing the amorphous silicon film at a comparative low temperature using a laser annealing.
For example, in a preparation of a polycrystalline silicon TFT with a bottom gate structure for LCD as shown in FIG. 1A-1D, it is known a laser annealing method in which an amorphous silicon film formed on a glass substrate is heated by irradiating an excimer laser to polycrystallize the amorphous silicon film.
In the preparation of the polycrystalline silicon TFT with a bottom gate structure, a Cr film is first formed on a glass substrate 10. After a certain pattern is formed on the film, a gate electrode 12 integral with a gate wiring is made. Next, as shown FIG. 1B, a gate insulating film 14 having a two layered structure and an amorphous silicon film 20 (which will be referred to as an a-Si film hereinafter) are successively formed with a plasma CVD (Plasma Enhanced Chemical Vapor Deposition).
Then, the a-Si film 20 is subjected to anneal by irradiating an excimer laser (ELA: Excimer Laser Annealing) to polycrystallize the formed a-Si film 20 and obtain a polycrystalline silicon 22 (which will be referred to as a p-Si hereinafter). The substrate temperature in this instance is normally a temperature ranging from a room temperature to about 300xc2x0 C.
After the p-Si film 22 is obtained through polycrystallization, a channel stopper film 30 consisting of SiO2 is formed at a region where a channel region 44 is to be formed (the region corresponding to the gate electrode 12) on the p-Si film 22 (See FIG. 1D). Next, an impurity (e.g. phosphorus) is doped to a region corresponding to a source drain region of a TFT by using a channel stopper 30 as a mask from above, as shown in the drawing. It should be noted that the TFT in FIG. 1D includes an LDD (Lightly Doped Drain) structure, that regions 42LS and 42LD are lightly doped source drain regions (Nxe2x88x92), and that regions 40S and 40D are heavily doped regions (N+) in the drawing.
After doping, these doped impurities are activated by a Rapid Thermal Annealing (RTA) with lamp annealing, thereby forming source and drain regions and a channel region in the p-Si film 22. After that, interlayer insulating films 50 and 52 are formed while a source electrode (which also acts as a source wiring in many cases) 70 is connected to the source region 40S. In case of a TFT for pixel portions in an LCD, a transparent conductive film of ITO (Indium Tin Oxide) which acts as a pixel electrode 60 is connected to the drain region 40D, thereby the other side of substrates for an LCD is obtained. A plan view of the TFT shown in FIG. 1D will be an arrangement as shown in FIG. 2. (It should be noted that FIG. 2 shows a status before the source electrode 70 and the pixel electrode 60 are formed.)
As outlined above, in a conventional polycrystalline silicon TFT with a bottom gate structure, the p-Si film 22 is obtained by polycrystallizing the a-Si film 20 with the ELA.
Since the polycrystallization of such an a-Si depends on the supplied amount of heat, i.e. the amount of energy, it is important to control the amount of heat so that it is uniformly supplied to an a-Si film. Namely, the energy of the excimer laser per unit area should be uniformly applied to the a-Si film in order to form a uniform p-Si film 22.
However, there is a drawback that the size of grains in the p-Si film 22 formed by the ELA are not uniform over all of the area.
The most critical reason why such non-uniformity occurs in the TFT with the bottom gate structure is that the a-Si film 20 to be polycrystallized is formed so as to cover the upper portion of the gate electrode 12 having a high thermal conductivity. Namely, the a-Si is formed to stride across the gate electrode 12 as shown in FIGS. 1A-1D or FIG. 2. The thermal conductivity of a metallic material (for example Cr) constituting the gate electrode 12 is higher than that of the other areas of the glass substrate 10 around the gate electrode 12. When the excimer laser is applied to the a-Si film 20, the heat provided by the excimer laser diffuses faster in a region of the a-Si film 20 under which the gate electrode 12 lies than in the other regions of the a-Si film 20 under which the glass substrate lies because of the existence of the gate electrode 12 and gate wiring.
For example, as shown in FIG. 3, the a-Si film 20 is formed with appropriate grain size polycrystalline silicon in an area 22Sub where the gate electrode 12 does not exist. By contrast, the a-Si film 20 is insufficiently polycrystallized in a region where the gate electrode 12 exists, thereby a polycrystalline silicon with an appropriate grain size can not be formed, even under the same annealing condition.
It may be considered that the condition for the laser annealing may be controlled so that grains of polycrystal in a region 22G sufficiently grow, because the region 22G, under which the gate electrode lies, in the p-Si film 22 formed by the polycrystallization constitutes a channel region of the TFT. However, if the condition for the annealing is set so that the grains in the region 22G of the silicon film under which the gate electrode 12 lies have an appropriate size, the size of polycrystalline grains in the region 22Sub must exceed an appropriate range, otherwise, the grains will become too small due to the oversupply of energy. This makes the characteristics of the region out of an allowable range. Accordingly, even if the annealing conditions are adjusted for the polycrystallization in the channel region, p-Si film with grains of an appropriate size can not be formed.
Furthermore, when the TFT is constituted of the p-Si film 22 with non-uniform grain sizes over a plane, as mentioned above, the characteristics of the respective TFT (e.g. an xe2x80x9conxe2x80x9d current and a sheet resistance depending on the grain size) varies widely. This causes a non-uniformity of the display and badly affects the quality of the display with the LCD when it is employed as a TFT for pixel portions in an LCD.
The present invention is made to resolve the above mentioned problems and it is an object of the present invention to form a uniform polycrystalline silicon by polycrystallizing an amorphous silicon. Furthermore, it is another object of the present invention to provide a thin film transistor with excellent characteristics by using polycrystalline silicon films of the invention.
The present invention is made to attain the above objects and characterized by the following.
A method of manufacturing a polycrystalline silicon film according to the present invention comprises steps of forming an amorphous silicon film so that it covers at least a portion of a material film which has a high thermal conductivity and is formed on a substrate, and polycrystallizing the amorphous silicon film by subjecting the amorphous silicon film to a lamp annealing process and a laser annealing process after the amorphous silicon film is formed to obtain a polycrystalline silicon film.
An amorphous silicon region under which the material film with a high thermal conductivity lies can be sufficiently heated by lamp annealing, thereby forming a polycrystalline silicon having an appropriate grain size on the material film over which it would be difficult to fully polycrystallize only the region using laser annealing. The constitution in which an amorphous film is formed above the high thermal conductivity material film may be applied to a TFT with a bottom gate structure, a semiconductor device with a multi-layer structure, or the like. By forming a polycrystalline silicon film from an amorphous silicon film according to the present method, it is possible to increase advantages of such TFTs or the semiconductor devices with a multi-layer structure.
Another aspect of the present invention relates to a method of manufacturing a thin film transistor with a bottom gate structure formed on a glass substrate and comprising the steps of forming an amorphous silicon film over a gate electrode film having a gate insulating film therebetween, and polycrystallizing the amorphous silicon film by subjecting the amorphous silicon film to a lamp annealing process and a laser annealing process after the amorphous silicon film is formed so as to form a polycrystalline silicon as an active layer of the thin film transistor.
Further, in a method of manufacturing a thin film transistor with a bottom gate structure formed on a glass substrate, the present invention may comprise the steps of polycrystallizing an amorphous silicon film by subjecting the amorphous silicon film to a lamp annealing process and a laser annealing process after the amorphous silicon film is formed as described above so as to form a polycrystalline silicon film, doping impurities to the polycrystalline silicon film, and activating the doped impurities by subjecting the polycrystalline silicon film to the laser annealing process and/or the lamp annealing process so as to form source and drain regions and a channel region of the thin film transistor in the polycrystalline silicon film.
When the polycrystalline silicon TFT with the bottom gate structure is manufactured, polycrystallines with an appropriate grain size can be obtained by subjecting the amorphous silicon in the active layer (a channel region) of the TFT formed over the gate electrode with the high thermal conductivity to the lamp annealing process and the laser annealing process. A polycrystalline silicon TFT with excellent characteristics can thus be obtained. In addition, since the polycrystallization under a low temperature becomes possible, the polycrystalline silicon film can be formed on a cheap glass substrate. The polycrystalline silicon TFT may be easily prepared with such films.
In addition, the present invention comprises the steps of subjecting an amorphous silicon film to a rapid thermal annealing process by applying a heat lamp and subjecting the amorphous silicon film to a laser annealing process by irradiating an excimer laser before or after (specially just after) the rapid thermal annealing process so as to polycrystallize the amorphous silicon film and to form the polycrystalline silicon film.
A polycrystalline silicon TFT formed by the above mentioned method can be easily used as a switching element for driving pixels of a matrix type display apparatus such as liquid crystal display apparatus and/or as a switching element for a driver circuit of a display apparatus. Especially, since a polycrystalline silicon TFT is obtained in accordance with the present invention, it becomes possible to form both switching elements for driving pixels and switching elements for a driver circuit of a display apparatus on one substrate in a same process, thereby reducing the cost of a display apparatus. In addition, since a driver circuit may be contained in the periphery of the panel, it is possible for the display apparatus to be made thinner and lighter. Also, the display quality of the display apparatus can be improved. These are advantageous factors as display size is increased.
In accordance with the present invention, an annealing apparatus for polycrystallizing an amorphous silicon film may comprise rapid thermal annealing processing portion for annealing an object to be annealed by applying halogen light and laser annealing processing portion for annealing the object to be annealed by applying an excimer laser.
When the annealing apparatus in which the rapid thermal annealing processing portion and the laser annealing processing portion are closely provided is employed, it is possible to form a polycrystalline silicon having uniform grains with an appropriate size from amorphous silicon at a low temperature. Also, since it is then possible to configure a simple annealing apparatus, labor can be significantly reduced.